Healthy mice cloned from frozen bodies

Healthy mice have been cloned for the first time from dead mice that had been frozen for several years, raising the possibility, scientists say, of "resurrecting" extinct animals such as mammoths from their frozen carcasses.

The clones were produced from dead mice kept at -20C for up to 16 years by a group of scientists in Kobe, Japan. After thawing the mice, researchers collected nuclei from cells in their brain tissue. These were then injected into empty eggs from which the DNA had been removed, to create cloned embryos. A second round of cloning used stem cells from the embryos that grew into four mouse clones.

Nine further clones were created by mixing the cells of different embryos, according to research published in the journal Proceedings of the National Academy of Sciences.

So-called "Dolly the sheep"-style cloning has previously been achieved using live donor cells from which DNA was transferred to eggs. Cloning from thawed frozen cells was thought to be difficult because ice crystals formed in frozen cells could damage the DNA, making cloning of long-dead animals impossible.

The scientists, led by Dr Teruhiko Wakayama from the Centre for Developmental Biology in Kobe, wrote: "We have demonstrated here that healthy cloned mice and chimeric clonal mice could be obtained by nuclear transfer using donor nuclei from cells obtained from bodies frozen without cryoprotectants for up to 16 years."

Other sources of frozen nuclei, such as white blood cells, could be as useful for cloning as brain tissue, said the scientists.

The research raises hopes that the cloning technique could be used to resurrect extinct animals frozen in permafrost, such as the woolly mammoth, Wakayama told New Scientist magazine.

"It would be very difficult, but our work suggests it is no longer science fiction," he said.